1. Field of the Invention
The present invention relates to a charging method and a charging apparatus suitable for charging a lithium-ion-based rechargeable battery.
2. Description of the Related Art
In recent years, a lithium-ion battery which is higher in energy density and lighter in weight than a nickel-cadmium battery and a nickel-hydride battery has been widespread as a rechargeable battery used for a power supply to a notebook-size personal computer, a mobile phone unit and the like.
Nickel-cadmium batteries and nickel-hydride batteries have a characteristic that the terminal voltage drops slightly in the fully charged state. Accordingly, as a method for charging those batteries, there has been employed a method in which the temporal change in terminal voltage is measured while charging the batteries with a constant current and the charging is finished by judging that the batteries are in full charge, when the amount of change becomes negative (when −ΔV is detected).
On the other hand, a terminal voltage does not drop even in a fully-charged state in the case of a lithium-ion battery. Therefore, when the lithium-ion battery is charged by such a method applied to the nickel-cadmium battery and the nickel-hydride battery, it is not possible to judge the fully-charged state and insufficient charging or overcharge occurs, which makes the performance thereof deteriorate. Hence, a method such as the following method (a) or method (b) has been employed in the past as a method for charging the lithium-ion battery.
(a) In the beginning stage of charging, a terminal voltage is measured while performing the charging with a constant current. Thereafter, when the terminal voltage reaches a predetermined voltage, the charging is changed to the one with a constant voltage and measurement of a charging current is started. Then, when the charging current becomes equal to or less than a predetermined reference value, it is judged that the battery is fully charged, and the charging is ended.
(b) In the beginning stage of charging, a terminal voltage is measured while performing the charging with a constant current. Thereafter, when the terminal voltage reaches a predetermined voltage, the charging with the constant current is performed intermittently. In the intermittent charging, the following processing is repeated. Specifically, after charging during a fixed period of time, the charging is halted until the terminal voltage decreases so as to be stabilized at approximately a fixed value, and then it is detected whether or not this stabilized voltage is equal to or more than the reference value (a value equal to or close to a full-charged voltage). Then, when the voltage stabilized during the halt of the charging becomes equal to or more than the above-described reference value, it is judged that the battery is fully charged, and the charging is ended (for example, refer to Patent reference 1).
[Patent reference 1] Published Japanese Patent Application No. H6-325795 (paragraph numbers 0013 through 0023, and FIGS. 1 through 5)
However, in the above method of performing the constant current charging followed by the constant voltage charging as described in (a), there has been an inconvenience that it takes a long time until charging is completed.
In addition, there also has been an inconvenience that it takes a long time until charging is completed in the method (b) described above, because the halt time becomes long at the time of performing the intermittent charging (charging is continuously halted until the terminal voltage is stabilized).